All organisms need energy to survive. While animals and fungi must obtain
this energy from other organisms (living or dead), plants can synthesize
energy-rich carbohydrates in the process of photosynthesis. Thus, plants
are referred to as "producers" because the virtually all large
organisms (other than microbes) depend on plants for food, either directly
or indirectly.

Some animals feed directly on the leaves of plants. Herbivores can range
is size from small insects

to large mammals.

Of course, there are other animals which rely on plant parts other than
leaves. Many animals include fruits, seeds, and/or nuts in their diets:

Many animals feed on nectar or pollen at flowers, especially insects.

Many decomposers obtain their nutrition from rotting plant material.

Many insects are primary consumers which depend on plants for their nutrition.
Insects themselves are preyed on by secondary consumers such as other
invertebrates,

fish,

amphibians,

reptiles,

birds,

and mammals.

Fish are serve as prey for other fish, amphibians,

reptiles,

birds, and mammals.

Amphibians can be eaten by a diversity of animals including fish, large
frogs, snakes, turtles, herons, raccoons. Young turtles are preyed upon
by a variety of animals including herons, foxes, and raccoons. Snakes
are eaten by larger snakes, hawks, and carnivorous mammals. Birds can
be preyed upon by snakes, hawks, falcons, cats, and foxes. Small mammals
compose part of the diet of snakes, hawks, and carnivorous mammals. Large
mammals can be eaten by coyotes and bears. Many large animals provide
nutrition to invertebrate parasites.

A food chain is a sum of predatory relationships. For example, one food
chain could begin with grass (the producer) which is eaten by a grasshopper
(the primary consumer) which is eaten by a garter snake (a secondary consumer)
which is eaten by a red-tailed hawk (a tertiary consumer). A different
food chain could begin with aquatic algae as producers which are eaten
by aquatic insect larvae which are eaten by fish which are eaten by a
river otter.
Food chains are important in our understanding of the environment. Any
change which affects one organism can indirectly affect the consumers
which feed on it and the higher level consumers which feed on these animals
and so on. Because pesticides are not broken down by the body, they tend
to accumulate at higher and higher concentrations in animals which are
higher and higher in the food chain. Thus, a pesticide which is present
in low concentrations in aquatic insects can accumulate to a much higher
concentration in fish-eating birds or humans which feed on fish.